Conventional filled polymer systems often have improved modulus, stiffness, and hardness relative to unfilled polymer systems. Use of nanofillers in polymers can improve the creep-resistance, wear-resistance, and modulus of the nanocomposite, without adversely affecting polymer aesthetics like clarity. Nanoparticles can also have a strong influence on the glass transition temperature (Tg) of polymers.
Although the high surface area of nanoparticles creates a large interface with host polymers, this high surface area also makes nanoparticles more prone to forming larger particles through agglomeration (a potentially reversible self-association that is frequently difficult and/or costly to reverse) or aggregation (an irreversible self-association). Agglomerated and aggregated nanoparticles frequently do not offer the level of benefits afforded by well-dispersed primary nanoparticles because they have less surface area in contact with the polymer matrix.
Colloidal silica is a potentially convenient source of nanoparticles (particles that are 100 nm in diameter or smaller) that might be blended with a polymer to improve various physical properties of the polymer. But colloidal silica can be difficult to disperse in solvents or polymers because the polar silanol groups on the surface of the nanoparticles can cause them to agglomerate. Even worse, the silanols can react chemically with each other (“condense”) and form irreversible linkages that cause the particles to irreversibly aggregate.
Attempts to overcome this tendency to agglomerate have included grafting polystyrene “brushes” onto the silica nanoparticle surface, but these modified particles are useful only for blends of polymers of the same s composition as the brushes, namely polystyrene. In addition, this approach uses an expensive multistep, reversible addition-fragmentation chain transfer polymerization process, with smelly sulfur reagents, to modify the surface.
Silanes can also be used to modify silica surfaces like glass, glass lo fibers, and fumed silica (aggregates of silica nanoparticles), but is rarely used with primary, unaggregated silica particles. Phenylsilane modification improves the compatibility and dispersibility of silica nanoparticles in non-polar aromatic polymers such as polystyrene. Similarly, perfluoroalkylethylsilanes can be used for fluoropolymers.
In colloidal silica (unaggregated silica nanoparticles suspended in a liquid medium), surface modification is not as facile as it is with glass or aggregated particles. It can adversely affect the stability of the nanoparticles and cause them to agglomerate or irreversibly aggregate, which leads to particle clusters that are not nanoparticles. This agglomeration or aggregation can also make the particles settle out or form a gel. These suspended particle clusters, settled particles, or gels cannot usually be well-dispersed in polymers.
There is a further need to modify the surface of colloidal particles with specific functional groups that interact with the polymers into which they are to be blended to improve the ability to disperse these particles throughout the host polymer without substantial agglomeration or aggregation. Better dispersion leads to fewer large particle agglomerates and aggregates and, therefore, better clarity, an important property for many product applications. Better dispersion also increases the interfacial area between particles and polymer, enhancing properties like wear-resistance and modulus. Better attachment of the particles to the polymer can increase the polymer's modulus and wear-resistance. Better dispersion can increase the viscosity and reduce the mobility of the polymer and thereby improve its resistance to creep.
Most common aminosilanes cannot be used to surface modify colloidal silica nanoparticles because they cause the nanoparticles to gel, agglomerate, or aggregate.
For example, 3-(aminopropyl)triethoxysilane (“APTES”), 4-(aminobutyl)triethoxysilane, and other primary aminoalkylsilanes have been used to surface-modify silica particles, where the particle size is 166 nm. 3-(Aminopropyl)triethoxysilane has been used to surface-modify silica gel particles of 60-125 microns in diameter. When APTES was used to lo surface-modify colloidal polypyrrole-silica particles of 113 nm in diameter, an increase in particle diameter after amination was noted, indicating some degree of flocculation. It has also been found that aminosilane modification of 100 nm colloidal silica using APTES causes flocculation, but that diethoxymethyl(aminopropyl)silane and monoethoxydimethyl(aminopropyl)silane give stable dispersions with no increase in particle size. Trialkoxysilanes are preferred over dialkoxyalkylsilanes and alkoxydialkylsilanes for surface modification because they react more rapidly than silanes with only one or two alkoxy groups.
Nevertheless, there remains a need for compositions comprising polymers and colloidal silica, with improved dispersion of the colloidal silica in the polymer.